In Cycles Preview (not rendering), GPU is 20 times faster than CPU, but only double in Rendering!
CPUs work better when using tiles, escpecially smaller tiles. While GPUs work better when using bigger tiles or in the case of preview, 1 tile the size of the whole image. So when previewing it's just like one big tile, and that's why the GPU is much faster. and when rendering, it uses small tiles, and that's why the CPU performs better when rendering.